# std::basic_ios

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< cpp‎ | io

Defined in header `<ios>` |  |
template< class CharT, class Traits = std::char_traits<CharT> > class basic_ios : public std::ios_base |  |

The class `std::basic_ios` provides facilities for interfacing with objects that have std::basic_streambuf interface. Several `std::basic_ios` objects can refer to one actual `std::basic_streambuf` object.

Inheritance diagram

Several typedefs for common character types are provided:

Defined in header `<ios>`
Type | Definition
`std::ios` | std::basic_ios<char>
`std::wios` | std::basic_ios<wchar_t>

### Member types

Member type | Definition
`char_type` | `CharT`
`traits_type` | `Traits`; the program is ill-formed if `Traits::char_type` is not `CharT`.
`int_type` | `Traits::int_type`
`pos_type` | `Traits::pos_type`
`off_type` | `Traits::off_type`

### Public member functions

(constructor) | constructs the object (public member function)
(destructor) [virtual] | destructs the object (virtual public member function)
operator= | the copy assignment operator is deleted (public member function)
##### State functions
good | checks if no error has occurred i.e. I/O operations are available (public member function)
eof | checks if end-of-file has been reached (public member function)
fail | checks if an error has occurred (public member function)
bad | checks if a non-recoverable error has occurred (public member function)
operator! | checks if an error has occurred (synonym of fail()) (public member function)
operator bool | checks if no error has occurred (synonym of `!`fail()) (public member function)
rdstate | returns state flags (public member function)
setstate | sets state flags (public member function)
clear | modifies state flags (public member function)
##### Formatting
copyfmt | copies formatting information (public member function)
fill | manages the fill character (public member function)
##### Miscellaneous
exceptions | manages exception mask (public member function)
imbue | sets the locale (public member function)
rdbuf | manages associated stream buffer (public member function)
tie | manages tied stream (public member function)
narrow | narrows characters (public member function)
widen | widens characters (public member function)

### Protected member functions

init | initializes a default-constructed std::basic_ios (protected member function)
move (C++11) | moves from another std::basic_ios except for `rdbuf` (protected member function)
swap (C++11) | swaps with another std::basic_ios except for `rdbuf` (protected member function)
set_rdbuf | replaces the `rdbuf` without clearing its error state (protected member function)

## Inherited from std::ios_base

### Member functions

##### Formatting
flags | manages format flags (public member function of `std::ios_base`)
setf | sets specific format flag (public member function of `std::ios_base`)
unsetf | clears specific format flag (public member function of `std::ios_base`)
precision | manages decimal precision of floating point operations (public member function of `std::ios_base`)
width | manages field width (public member function of `std::ios_base`)
##### Locales
imbue | sets locale (public member function of `std::ios_base`)
getloc | returns current locale (public member function of `std::ios_base`)
##### Internal extensible array
xalloc [static] | returns a program-wide unique integer that is safe to use as index to `pword()` and `iword()` (public static member function of `std::ios_base`)
iword | resizes the private storage if necessary and access to the long element at the given index (public member function of `std::ios_base`)
pword | resizes the private storage if necessary and access to the void* element at the given index (public member function of `std::ios_base`)
##### Miscellaneous
register_callback | registers event callback function (public member function of `std::ios_base`)
sync_with_stdio [static] | sets whether C++ and C I/O libraries are interoperable (public static member function of `std::ios_base`)
##### Member classes
failure | stream exception (public member class of `std::ios_base`)
Init | initializes standard stream objects (public member class of `std::ios_base`)

### Member types and constants
Type | Explanation
openmode | stream open mode type The following constants are also defined:
Constant | Explanation
`app` | seek to the end of stream before each write
`binary` | open in binary mode
`in` | open for reading
`out` | open for writing
`trunc` | discard the contents of the stream when opening
`ate` | seek to the end of stream immediately after open
`noreplace` (C++23) | open in exclusive mode
(typedef)
fmtflags | formatting flags type The following constants are also defined:
Constant | Explanation
`dec` | use decimal base for integer I/O: see std::dec
`oct` | use octal base for integer I/O: see std::oct
`hex` | use hexadecimal base for integer I/O: see std::hex
`basefield` | dec | oct | hex. Useful for masking operations
`left` | left adjustment (adds fill characters to the right): see std::left
`right` | right adjustment (adds fill characters to the left): see std::right
`internal` | internal adjustment (adds fill characters to the internal designated point): see std::internal
`adjustfield` | left | right | internal. Useful for masking operations
`scientific` | generate floating point types using scientific notation, or hex notation if combined with fixed: see std::scientific
`fixed` | generate floating point types using fixed notation, or hex notation if combined with scientific: see std::fixed
`floatfield` | scientific | fixed. Useful for masking operations
`boolalpha` | insert and extract bool type in alphanumeric format: see std::boolalpha
`showbase` | generate a prefix indicating the numeric base for integer output, require the currency indicator in monetary I/O: see std::showbase
`showpoint` | generate a decimal-point character unconditionally for floating-point number output: see std::showpoint
`showpos` | generate a + character for non-negative numeric output: see std::showpos
`skipws` | skip leading whitespace before certain input operations: see std::skipws
`unitbuf` | flush the output after each output operation: see std::unitbuf
`uppercase` | replace certain lowercase letters with their uppercase equivalents in certain output operations: see std::uppercase
(typedef)
iostate | state of the stream type The following constants are also defined:
Constant | Explanation
`goodbit` | no error
`badbit` | irrecoverable stream error
`failbit` | input/output operation failed (formatting or extraction error)
`eofbit` | associated input sequence has reached end-of-file
(typedef)
seekdir | seeking direction type The following constants are also defined:
Constant | Explanation
`beg` | the beginning of a stream
`end` | the ending of a stream
`cur` | the current position of stream position indicator
(typedef)
event | specifies event type (enum)
event_callback | callback function type (typedef)

### Notes

Straightforward implementation of `std::basic_ios` stores only the following members (which all depend on the template parameters and thus cannot be part of std::ios_base):

- the fill character (see `fill()`)
- the tied stream pointer (see `tie()`)
- the associated stream buffer pointer (see `rdbuf()`).

Actual implementations vary:

Microsoft Visual Studio stores just those three members.

LLVM libc++ stores 1 less member: it maintains the `rdbuf` pointer as a void* member of `ios_base`.

GNU libstdc++ stores 4 additional members: three cached facets and a flag to indicate that fill was initialized.

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